1. Field of the Invention
The present invention relates to a temperature detection circuit for compensating an operation of, for example, an oscillator and protecting a semiconductor device from thermal breakdown. In particular, the invention relates to a temperature detection circuit capable of suppressing variations in temperature to be detected.
2. Description of Related Art
FIG. 5 is a circuit diagram showing a conventional temperature detection circuit. As shown in FIG. 5, a conventional temperature detection circuit 101 includes a temperature sensor potential generating part 120 and an inverter circuit 110. The temperature sensor potential generating part 120 is connected between a circuit power source VI and a ground GND to generate a potential according to an environmental temperature. The inverter circuit 110 detects a predetermined temperature based on the potential generated in the temperature sensor potential generating part 120.
The temperature sensor potential generating part 120 includes a resistor R12 and plural diodes D11 to D18 series-connected with the resistor R12 (hereinafter, referred to as cascaded diodes 121). The resistor R12 and the cascaded diodes are connected between the circuit power source VI and the ground GND.
Further, the inverter circuit 110 includes a resistor R11 and an NchMOSFET M11 series-connected with the resistor R11. The resistor R11 and the NchMOSFET M11 are connected between the circuit power source VI and the ground GND.
A node (detection node) N11 between the resistor R12 of the temperature sensor potential generating part 120, and the cascaded diodes 121 is connected with a gate of the NchMOSFET M11 of the inverter circuit 110. Thus, a temperature sensor potential VF at the detection node N11 is applied to the gate of the NchMOSFET M11. A current flowing through the resistor R12 of the temperature sensor potential generating part 120 flows through the cascaded diodes 121 as a constant current. The temperature detection circuit 101 is a temperature sensor that measures a forward voltage drop of the cascaded diodes 121 to thereby detect a temperature change.
FIG. 6 is a graph showing a temperature characteristic of the conventional temperature detection circuit 101 of FIG. 5. In FIG. 6, the vertical axis represents a potential (V), and the horizontal axis represents a temperature (° C.). The cascaded diodes 121 are such that 8 diodes are cascaded. As shown in FIG. 6, the temperature sensor potential VF generated in the cascaded diodes 121 shows negative temperature characteristics. Meanwhile, a threshold voltage Vth of the inverter circuit 110 composed of the resistor R11 and the NchMOSFET M11 shows positive temperature characteristics. The temperature detection circuit 101 detects the temperature at a potential Vdet at a point where characteristic lines of the temperature sensor potential VF and the threshold voltage Vth of the inverter circuit 110 cross each other, as a detection temperature Tdet. That is, when the temperature sensor potential VF reaches or exceeds the threshold voltage Vth of the inverter circuit 110, a detection signal is output from an output VO.
Referring now to FIG. 6, a variation in threshold voltage Vth of the inverter circuit 110 leads to a variation ΔT of the detection temperature Tdet as indicated by the detection potentials Vdet1 and Vdet2. Hence, it is difficult for the conventional temperature detection circuit 101 to accurately detect the temperature. For example, Japanese Unexamined Patent Application Publication No. 2001-13011 (pp. 2-3, FIG. 1) discloses the following temperature detection circuit. That is, the temperature detection circuit disclosed in this publication aims at detecting the same temperature irrespective of various variable factors in a manufacturing process for a device generating an electric signal in accordance with the environmental temperature. This circuit converts the electric signal generated in accordance with the temperature into digital data and then corrects the data. By correcting the digital data, the temperature detection circuit disclosed in this publication can give substantially the same digital output value under the same temperature.
Incidentally, a control circuit and protective circuit for currently popular IPDs (Intelligent Power Device) are made up of a CMOS. The IPD implies a high-performance integrated circuit imparted with various functions, in which a power device adapted to a large current and high voltage, and its control circuit and protective circuit are integrated.
In recent years, in an integrated circuit such as the IPD, a PchMOSFET is omitted for cost reduction, thereby lowering a process cost, and only an NchMOSFET is used to compose the circuit, so the control circuit and the protective circuit are simplified. In this case, when the circuit is configured by the NchMOSFET alone, it is difficult to configure a complicated circuit such as a comparator circuit. Hence, as shown in FIGS. 5 and 6, there is no choice but to adopt the circuit configuration utilizing the threshold voltage of the NchMOSFET. The temperature detection circuit is used for detecting, for example, an overheat temperature of the circuit. A variation in threshold voltage of the NchMOSFET largely affects the detection temperature of the temperature detection circuit. In other words, the threshold voltage Vth of the inverter circuit 110 depends on the threshold voltage Vth of the NchMOSFET M11, so the manufacturing variation in threshold voltage Vth increases the variation in detection temperature.
In contrast, in order to adjust the detection result with an aim to reduce the variation in detection temperature as in the temperature detection circuit disclosed in Japanese Unexamined Patent Application Publication No. 2001-13011, for example, it is necessary to store correction data according to each circuit. Further, there is a need for an arithmetic circuit for computing corrected data from the correction data based on the detection data. The more complicated process for manufacturing the detection circuit leads to an increase in circuit size, resulting in a higher manufacturing cost rather than cost reduction.